1. The Field of the Invention
The present invention relates to the formation of aluminum films. More particularly, the present invention relates to the formation of an aluminum film with a reduced occurrence and size of voids in a process suitable for forming contacts, vias, and metal interconnect lines situated on a semiconductor substrate assembly.
2. The Relevant Technology
Integrated circuits are manufactured by an elaborate process in which a variety of different semiconductor devices are integrally formed on a small silicon wafer. Conventional semiconductor devices include capacitors, resistors, transistors, diodes, and the like. In advanced manufacturing of integrated circuits, hundreds of thousands of semiconductor devices are formed on a single wafer.
One step in the manufacture of integrated circuits is to form metal interconnect lines between the discrete semiconductor devices on the integrated circuit. The metal interconnect lines allow for an electrical current to be delivered to and from the semiconductor devices so that the integrated circuit can perform its intended function.
The metal interconnect lines generally comprise narrow lines of aluminum etched from an aluminum film. Aluminum is typically used because it has relatively low resistivity, good current-carrying density, superior adhesion to silicon dioxide, and is available in high purity. Each of these properties is desirable in contact lines since they result in a faster and more efficient electronic circuit.
A further step in which aluminum films are used and which is frequently conducted in the manufacturing of integrated circuits is the formation of inter-level interconnect structures between nonadjacent layers on the integrated circuit wafer. These inter-level interconnect structures include contacts, plugs, and vias. An example of an interconnect structure in the form of a contact is shown in FIG. 1. In FIG. 1, a contact is shown with a contact opening 14 extending through a passivation layer 12 down to a silicon substrate assembly 10. A substrate assembly is intended herein to mean a substrate having one or more layers or structures thereon. Contact opening 14 is filled with a titanium nitride diffusion barrier 15 and a film of aluminum 16.
One complication arising from the use of aluminum films is the phenomenon referred to as xe2x80x9cvoid formation.xe2x80x9d Void formation is a process in which minute voids 18, shown in FIG. 1, form between grain boundaries as grains move and grow as a result of heat treatments following metal deposition. Void formation is believed to stem from the solid phase growth of aluminum crystals as the film becomes thicker during deposition. Crystal growth causes grain boundary movement and gives rise to voids and notches at the interface between two adjacent grains. The voids, also known as xe2x80x9cnotches,xe2x80x9d coalesce at flux divergence sites such as grain boundary triple points of a metal interconnect line, contact, or via. Voids cause a heightened resistivity in the aluminum film, thereby reducing the effectiveness of the aluminum film in providing electrical communication.
Void formation is accompanied by a phenomenon known as electromigration. Electromigration occurs as an electrical current flows through an aluminum portion of an interconnect line, contact, via, or other structure. When a voltage is applied across the aluminum, electrons begin to flow through the aluminum. These electrons impart energy to the aluminum atoms sufficient to eject aluminum atoms from their lattice sites. As the aluminum atoms become mobile, they leave behind vacancies. In turn, the vacancies are also mobile, since they can be filled by other aluminum atoms which then open new vacancies. In the phenomenon of electromigration, the vacancies formed throughout the aluminum line tend to coalesce at the grain boundary triple points of the metal line, thereby also helping to form voids.
In a further complication, voids 18 tend to coalesce due to increased movement of the grains at higher temperatures to form large-scale voids 20, as shown in FIG. 2. Large-scale voids 20 severely limit conductivity and can cause failure of the interconnect, via, or contact. This can result in the failure of the semiconductor device as a whole and failure of the entire integrated circuit. This problem occurs more frequently and is increasingly problematic at greater miniaturization levels due to the smaller size of the structures in relation to large scale voids.
In one attempt to eliminate void formation, aluminum is mixed with other metals to form an aluminum alloy. For example, copper is frequently added to aluminum. The addition of copper increases the energy required to cause the voids to form in the metal interconnect line. Titanium and other metals are also frequently deposited together with the aluminum, which metals are then alloyed to the aluminum in a high temperature anneal process flow step. Alloying is only a partial remedy, however, as void formation still continues to occur over time, especially as the size of aluminum features decrease.
To ensure adequate coverage of aluminum films, which is desirable in filling high aspect ratio via and contact holes, and in maintaining low contact resistance and consistent etching, the prior art has turned to cold wall CVD aluminum deposition processes. One example of this is organometallic chemical vapor deposition (MOCVD) using dimethyl aluminum hydride (DMAH). When so doing, a base layer of titanium nitride is typically deposited by chemical vapor deposition prior to the deposition of the aluminum film. The base layer of titanium nitride assists in the process of achieving uniform nucleation of the aluminum film. While this process is somewhat beneficial, the deposited aluminum films continue to exhibit voids 18 in aluminum film 28 and at the aluminum-titanium nitride interface due to grain growth, an illustration of which appears in FIG. 3.
FIG. 3 shows a substrate 10 subjacent to a passivation layer 22 composed of, for example, BPSG or SiO2. A titanium nitride base layer 26 is formed over passivation layer 22 and is subjacent an aluminum film 28. A plurality of voids 18 have formed in aluminum film 28 and at an interface 24 between titanium nitride base layer 26 and aluminum film 28. Voids 18 result in increased resistance in aluminum film 28, reliability problems of integrated circuits being formed by the structure of FIG. 3, and ultimately device failure that results in lowered yield rates of the process by which the integrated circuits are manufactured.
As a result of the above discussion, it is apparent that a need exists in the art for a method of aluminum film deposition which can be used to form uniform aluminum films with substantially eliminated or arrested void formation. Such a method would be additionally beneficial if it could be used for forming aluminum interconnect lines and inter-level interconnect structures, if it could be used with alloying processes, and if it could provide reduced electromigration.
In accordance with the invention as embodied and broadly described herein in the preferred embodiment, a method is provided for depositing aluminum films with a reduced occurrence and size of voids and notches. The method comprises in a first step, forming an underlying layer. One preferred underlying layer is titanium nitride which is deposited in a conventional manner.
In a further step, aluminum of a predetermined first thickness is deposited over the titanium nitride underlayer. The first thickness of aluminum is typically between about one third and one half of the total desired thickness of the aluminum film being deposited. The aluminum is preferably deposited with a cold wall chemical vapor deposition process, but could be deposited using any suitable process. The surface of the first thickness of aluminum is then exposed to a passivation species which coats the aluminum grain and precipitates at the grain boundaries. The passivation species comprises compounds such as O2, N2, TiCl4, TDMAT, SiH4, GeH4, as well as members of the halogen and interhalogen families of gases. Other species which achieves surface passivation can also be used.
In a further step, an aluminum film having a second thickness is deposited over the aluminum film having the first thickness. In the basic embodiment, the second thickness of the later deposited aluminum film comprises the remainder of the final thickness of the aluminum film desired to be formed.
The passivation species to which the surface of the aluminum film having the first thickness is exposed causes friction between the aluminum grains, both chemically and mechanically, so as to arrest and make the grains of the aluminum at the surface immobile. Immobilizing the grains causes a reduction in movement across grain boundaries and a suppression of grain growth. This in turn inhibits voids from forming and prohibits voids that do form from coalescing to form large-scale voids. The result is an aluminum film with a reduced amount of voids and notches from the prior art and with a reduced coalescence of the voids that are present.
In a second embodiment, the passivation species is deposited with CVD as organometallic chemical vapor deposition (MOCVD). The passivation species used in the second embodiment may comprise, for instance, Ti, TiN, TiCxNy, TiCxNyOz, Cu, Sc, C, Si, Ge, and Sb. The MOCVD deposited passivation species achieves surface passivation as described above, and in addition, forms an alloy with the adjoining aluminum films, thereby improving electromigration resistance.
A third embodiment involves multiple depositions of aluminum. Under this embodiment, an aluminum film having a first thickness is deposited and then passivated in the manner discussed above. An aluminum film having a second thickness is deposited and is also passivated. Additional aluminum films are then subsequently deposited, each having a predetermined thicknesses. A layer of passivation species is deposited after each aluminum film deposition, and the process is repeated until the final desired thickness of a resultant composite aluminum film is obtained. Additionally, each subsequent deposition is preferably conducted at a successively lower temperature, further helping to suppress void formation and to reduce coalescence of voids into large-scale voids.
An aluminum film structure is produced by the method of the present invention, having a substrate assembly and a plurality of adjacent layers situated upon the substrate assembly. Each adjacent layer has a substantially uniform thickness and an upper surface. The upper surface has a plurality of aluminum grains that are substantially immobile such that the grain boundary movement thereof is substantially arrested. Each adjacent layer has substantially no voids larger than the thickness thereof.
The aluminum film produced by the method of the present invention is highly suitable for forming metal interconnect lines and for forming inter-level interconnect structures down to semiconductor devices on a substrate assembly. The aluminum film has less voids and notches than if the passivation species were not used, and any voids and notches that do exist will not cross over the boundaries between the discrete layers laid by the multiple deposition steps. This in turn limits the maximum diameter of voids that can occur to the thickness of any of the discrete layers.
The above discussion illustrates several embodiments of a method for forming aluminum films. The method is suitable for use in forming aluminum interconnect lines, contacts, vias, and for other aluminum film applications. The method provides an interface with reduced occurrence and size of voids and notches, reduced electromigration, and increased conductivity. Faster device speeds and improved yield rates are also a result of the inventive method.